fix merge property in rare cases

Signed-off-by: Zhiyuan Chen <[email protected]>

chanfig/nested_dict.py

@@ -698,8 +698,7 @@ def _merge(this: FlatDict, that: Iterable, overwrite: bool = True) -> Mapping:
             return this
         if isinstance(that, Mapping):
             that = that.items()
-        context = this.converting() if isinstance(this, NestedDict) else nullcontext()
-        with context:
+        with this.converting() if isinstance(this, NestedDict) else nullcontext():
             for key, value in that:
                 if key in this and isinstance(this[key], Mapping):
                     if isinstance(value, Mapping):
@@ -709,13 +708,21 @@ def _merge(this: FlatDict, that: Iterable, overwrite: bool = True) -> Mapping:
                             this.set(key, value)
                         else:
                             this[key] = value
-                elif key in dir(this) and isinstance(getattr(this.__class__, key), (property, cached_property)):
-                    getattr(this, key).merge(value, overwrite=overwrite)
+                elif key in dir(this) and isinstance(getattr(this.__class__, key, None), (property, cached_property)):
+                    if isinstance(getattr(this, key, None), FlatDict):
+                        getattr(this, key).merge(value, overwrite=overwrite)
+                    else:
+                        setattr(this, key, value)
                 elif overwrite or key not in this:
                     if isinstance(this, NestedDict):
                         this.set(key, value)
                     else:
                         this[key] = value
+                else:
+                    raise ValueError(
+                        f"""{key} is not merged.
+                        This is likely to be a bug, please report at https://github.com/ZhiyuanChen/CHANfiG/issues."""
+                    )
         return this
 
     def intersect(self, other: Mapping | Iterable | PathStr, recursive: bool = True) -> Self:  # pylint: disable=W0221